Backrest of a chair



FIG. 1 is a three-quarter perspective of backrest of a chair showing my new design;

FIG. 2 is a three-quarter perspective view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is a front view thereof;

FIG. 5 is a rear view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof. 

The ornamental design for a backrest of a chair, substantially as shown and described. 